Revert freeform-specific disconnect logic in core.

Since Shell now controls the freeform-specific logic for disconnect transitions, we no longer need any of the specific logic implemented in core. Not only are these changes unneeded, but they also break CTS, so this CL reverts them.

Bug: 391652399
Test: Manual, atest
Flag: com.android.window.flags.enable_display_disconnect_interaction
Change-Id: I14f0dfdd47a0af65bc64b8766329a8047cb1a409